1. Technical Field
The present invention relates to a heat sink and a process for producing the same. More particularly, the present invention relates to a heat sink having such a laminate structure that a layer of diamond of high thermal conductivity is superimposed on a substrate of aluminum nitride of high thermal conductivity, and further relates to a process for producing the heat sink.
2. Description of the Prior Art
The processing capacity of electronic components is making striking enhancement in accordance with the increase of information density. Thus, currently a large amount of heat is emitted from each electronic component. Since maintaining constant temperature is desired for ensuring the stable functioning of electronic components, various improvements for cooling them have been proposed. It is common practice to, in practical use, mount electronic components tending to have high temperature on a heat radiating member, namely, a material capable of absorbing heat, or a device including such a material for thermally protecting a constituent element or a system known as a heat sink.
As materials having been in practical use in heat sinks for long, there can be mentioned metals and metal alloys of satisfactory thermal conductivity, such as Cu and Cuxe2x80x94W, and semiconductive or insulating ceramics of high thermal conductivity, such as SiC and AlN. However, as a result of integration of electronic components, the outgoing heat is increasing to such an extent that the cooling with the use of a heat sink based on these materials only encounters a limit. Consequently, a heat sink material based on diamond exhibiting the highest thermal conductivity (about 2000 W/mK) among the existent materials, has been developed as a new heat sink material for enhancing heat radiation characteristics. As such a heat sink which is now common, there can be mentioned a substrate of copper and, brazed thereto, a single crystal diamond in plate or film form, known as a submount. However, the heat radiation characteristics of this heat sink have not always been satisfactory because the single crystal diamond is so expensive that a heat sink of large configuration cannot be employed and because the brazing material becomes a resistor to heat conduction.
Therefore, forming of a polycrystalline diamond film on a substrate according to the vapor phase synthetic process has been tried. For example, Japanese Patent Laid-open Publication No. 5(1993)-13843 proposed a heat radiating member consisting of a substrate having a surface for mounting a semiconductor element thereon and, covering the mounting surface, a diamond layer synthesized by the vapor phase process. This heat radiating member is for the purpose of suppressing the deterioration of performance of semiconductor laser elements by generated heat, and obtained by directly forming a polycrystalline diamond layer of 10 to 500 xcexcm thickness on a substrate in accordance with the microwave plasma CVD (Chemical Vapor Deposition) process. It would be feasible to use this heat radiating member as a submount by miniaturizing the same.
In the above heat sink, the most suitable material of substrate on which a polycrystalline diamond film is to be formed varies depending on the usage. Among various substrate materials, sintered aluminum nitride (AlN) is known as a material having not only insulating properties but also high thermal conductivity. The AlN substrate is especially useful when it is intended to form a circuit on a substrate. Specifically, when it is intended to form a polycrystalline diamond layer only on an area of substrate on which an element is to be mounted while forming a circuit on areas of substrate other than the area of substrate on which an element is to be mounted, an insulating material such as a ceramic is preferred as a substrate material from the viewpoint that additional formation of an insulating film is not needed. Moreover, a material of high thermal conductivity is desired for avoiding the drop of heat radiation efficiency, namely, maintaining high thermal conductivity with respect to the whole body of substrate. Therefore, the use of sintered aluminum nitride (AlN) as a substrate material having not only insulating properties but also high thermal conductivity is to be considered.
In this connection, in the use of a metal of high thermal conductivity as a substrate, it is possible to obtain a substrate having not only insulating properties but also high thermal conductivity by laminating the substrate with an insulating film of, for example, SiO2 in accordance with the vapor deposition method or the like. However, this insulating film often poses a problem with respect to the reliability in voltage withstanding properties, etc.
It is anticipated that the above heat sink comprising a substrate of sintered aluminum nitride (AlN) and, superimposed thereon, a polycrystalline diamond layer will have high availability.
However, it is difficult to directly form a polycrystalline diamond layer of high quality on a substrate of sintered aluminum nitride (AlN) in accordance with the vapor phase synthetic process. From the practical viewpoint, any heat sink comprising a polycrystalline diamond layer of high thermal conductivity superimposed on a substrate of AlN is not known. For example, although the above Japanese Patent Laid-open Publication No. 5(1993)-13843 describes that Cu, Cuxe2x80x94W alloy, Cuxe2x80x94Mo alloy, Cuxe2x80x94Wxe2x80x94Mo alloy, W, Mo, sintered SiC, sintered Si3N4, sintered AlN and the like can be used as substrate materials, the thermal conductivity of the polycrystalline diamond actually formed on a substrate of sintered AlN is as extremely low as 300 W/mxc2x7K.
It is an object of the present invention to provide a method of forming a polycrystalline diamond layer of high quality on a ceramic substrate containing aluminum nitride (AlN) as a principal component. It is another object of the present invention to provide a heat sink having such a fundamental structure that a polycrystalline diamond film is formed on the above substrate, and having excellent heat radiation characteristics.
The present inventor has found that a polycrystalline diamond film of high quality exhibiting high thermal conductivity can be produced on a ceramic substrate containing aluminum nitride as a principal component by preliminarily forming thereon a layer of specified substance exhibiting excellent adhesion to the substrate and a diamond film and thereafter forming a polycrystalline diamond film on the above layer in accordance with the vapor phase method.
Therefore, according to one aspect of the present invention, there is provided a heat sink comprising a ceramic substrate having at least one plane containing aluminum nitride as a principal component, and a diamond film layer superimposed on the plane of the ceramic substrate, characterized in that the ceramic substrate and the diamond film layer are bonded together via a bonding member interposed therebetween.
In the above heat sink of the present invention, a ceramic substrate containing, as a principal component, aluminum nitride which is excellent in insulating characteristics and heat radiation characteristics is used as a substrate material. Accordingly, the heat absorption performance and heat radiation performance of the heat sink as a whole are excellent. Further, in case of the heat sink having such a structure that the diamond film is formed so as not to cover the entire surface of the substrate, an electronic circuit made of a metal can be printed on the substrate without the particular need to form an insulating film.
In the heat sink of the present invention, it is preferred that the bonding member be constituted of at least one material selected from the group consisting of silicon, silicon carbide, tungsten, tungsten carbide, CuW, Cuxe2x80x94Mo alloy, Cuxe2x80x94Moxe2x80x94W alloy, amorphous carbon, boron nitride, carbon nitride and titanium. In the use of this bonding member, cracking or other faults of the diamond film superimposed on the bonding member can be avoided, the heat radiation performance is enhanced, and the durability of the diamond film is prolonged.
This bonding member may be constituted of a crystalline substance orientated on a specified crystal face. In particular, it is preferred that the bonding member be constituted of a polycrystalline silicon preferentially orientated for crystal face (111), crystal face (220) or crystal face (400.) When the bonding member is constituted of a crystalline substance, the crystal grains of the polycrystalline diamond film formed on the bonding member would be enlarged and the crystallinity thereof would be high, so that the diamond film would have especially high thermal conductivity.
According to necessity, the polycrystalline silicon may contain a dopant. The addition of the dopant would enable enhancing the orientation of polycrystalline silicon and would also enable imparting electrical conductivity to the silicon film.
In the heat sink of the present invention, preferably, the diamond film layer exhibits a thermal conductivity of 800 W/mK or greater. The heat sink of the present invention attains enhanced thermal conductivity as compared with that of a heat sink wherein a diamond film layer is directly superimposed on a substrate.
According to another aspect of the present invention, there is provided a process for producing a heat sink, comprising the steps of providing a ceramic substrate having at least one plane containing aluminum nitride as a principal component; forming a bonding member layer on at least a portion of the plane of the ceramic substrate; and forming a diamond film on the bonding member layer.
In the above process of the present invention, peeling or cracking of the diamond film would not occur during the cooling step after the diamond film formation step wherein the substrate is maintained at 700 to 1100xc2x0 C. Thus, a heat sink of high quality according to the present invention can be stably produced in high efficiency in the process of the present invention. In particular, when a layer of crystalline substance orientated for a specified crystal face is formed as the bonding member layer, a diamond film of large crystal grains and high crystallinity can be easily formed. Moreover, when a layer containing silicon as a principal component is formed as the bonding member layer, a diamond nucleation would be promoted, thereby enabling formation of a diamond film in higher efficiency. Furthermore, in the process wherein the bonding member layer is formed from a conductive substance, such as a polycrystalline silicon containing a dopant, and wherein, while a direct current voltage is applied to the bonding member layer, a thin diamond film is formed on the bonding member layer in accordance with microwave CVD or hot filament CVD method, it would be feasible to efficiently deposit an effective precursor for diamond film synthesis on the substrate surface, thereby easily forming a diamond film of high crystallinity, high orientation degree and high thermal conductivity.